GNSS (Global Navigation Satellite Systems) such as a GPS (Global Positioning System) are used for, for example, positioning by receiving GNSS signals broadcasted from positioning satellites. Each GNSS signal is comprised of a spread spectrum signal which is code modulated by a pseudo noise.
When receiving such a GNSS signal, if a signal other than the GNSS signal (hereinafter, referred to as the interference wave signal) is received, a disadvantage may be caused, for example, reception sensitivity to the GNSS signal degrades.
Therefore, Patent Document 1 and Patent Document 2 disclose interference wave signal removing devices for detecting and removing interference wave signals of which frequency bands are narrow (having narrow bands) different from GNSS signals. FIG. 1 is a block diagram of main circuits of the conventional interference wave signal removing device 100P disclosed in Patent Document 1.
The conventional interference wave signal removing device 100P disclosed in Patent Document 1 includes the controller 101P, the notch filter 102P, the frequency analyzer 103P, and the frequency scanner 104P. The controller 101P specifies a frequency of an interference wave signal based on a frequency spectrum of an input signal Si obtained from the frequency analyzer 103P and a frequency spectrum of an output signal Sop obtained from the frequency scanner 104P. Further specifically, the specification of the interference wave signal frequency is performed by the following processing.
FIG. 2 is a view illustrating a concept of a plurality of frequency BIN set by the conventional frequency scanner 104P. As illustrated in FIG. 2, the frequency scanner 104P divides an entire scan frequency BWfA into a plurality (5,000 in FIG. 2) of frequency BIN each comprised of a frequency band BWfABIN, integrates signals per unit of the frequency BIN, and outputs the integrated signals of the respective frequency BIN to the controller 101P. The controller 101P detects the frequency BIN of the integrated signals comprised of signal levels above a predetermined threshold and sets the central frequency of the frequency BIN to the interference wave signal frequency.
The controller 101P adjusts the attenuation property of the notch filter 102P to attenuate the interference wave signal frequency based on the information of the specified interference wave signal frequency.
Since the interference wave signal frequency is specified by such processing, in the method of Patent Document 1, the frequency resolution for detecting the interference wave signal frequency is determined by the bandwidth of the frequency BIN. Therefore, if the bandwidth of the frequency BIN is wide, the interference wave signal frequency cannot be detected in high accuracy, and if the bandwidth of the frequency BIN is narrow, even though the interference wave signal frequency can be detected in high accuracy, the number of the frequency BIN increases and the detecting time length of the interference wave signal frequency increases. For example, when the bandwidth of the frequency BIN is set to 1/N, if the entire scan frequency band range is stable, the number of the frequency BIN to be scanned increases by N-times and the integrating time length for one frequency BIN increases by N-times. Therefore, the scanning period of time for the entire scan frequency band range increases by N2-times.
Therefore, in Patent Document 2, the interference wave signal frequency is estimated by using a concept illustrated in FIG. 3. FIG. 3 is a view for describing a frequency estimation concept in Patent Document 2. In FIG. 3, FS[f(n)] indicates a sinc function of the of the frequency BIN of which the central frequency is f(n). FS[f(n+1)] indicates a sinc function on the higher frequency side by one frequency BIN from the frequency BIN of FS[f(n)], where the central frequency is f(n+1). FS[f(n−1)] indicates a sinc function on the lower frequency side by one frequency BIN from the frequency BIN of FS[f(n)], where the central frequency is f(n−1). Here, when the integrating time period is T, the frequency bandwidth BW of each frequency BIN is 1/T, and the central frequency f(n+1)=f(n)+1/T and the central frequency f(n−1)=f(n)−1/T.
In Patent Document 2, by using the setting of the frequency BIN illustrated in FIG. 2, the signal level ZCW(n) of the frequency BIN of FS[f(n)] where the interference wave signal is detected, and the signal level ZCW(n+1) of the frequency BIN of FS[f(n+1)] or the signal level ZCW(n−1) of the frequency BIN of FS[f(n−1)], an estimation of an interference wave signal frequency fCW is calculated based on Equation (1) if ZCW(n+1)>ZCW(n−1) or based on Equation (2) if ZCW(n+1)<ZCW(n−1).
                              f          CW                =                              f            ⁡                          (              n              )                                +                                    1              T                        ⁢                                                            Z                  cw                                ⁡                                  (                                      n                    +                    1                                    )                                                                                                  Z                    cw                                    ⁡                                      (                    n                    )                                                  +                                                      Z                    cw                                    ⁡                                      (                                          n                      +                      1                                        )                                                                                                          (        1        )                                          f          CW                =                              f            ⁡                          (              n              )                                -                                    1              T                        ⁢                                                            Z                  cw                                ⁡                                  (                                      n                    -                    1                                    )                                                                                                  Z                    cw                                    ⁡                                      (                    n                    )                                                  +                                                      Z                    cw                                    ⁡                                      (                                          n                      -                      1                                        )                                                                                                          (        2        )            